In recent years, the use of disposable diapers in place of reusable cloth diapers has become commonplace, and the market share for disposables is continuing to grow. Disposable diapers commonly comprise a fluid-absorbent pad disposed between a fluid-pervious facing sheet and a thin, flexible, fluid-impervious backing sheet. Most disposable diapers are also usually equipped with a pair of pressure-sensitive tapes which are used to fasten the diaper about the waist of the wearer. When the diapers are manufactured, one end of each tape is permanently secured to a thin film backing sheet near a corner of the diaper while the other free end of the tape is covered by a protective release sheet, which sheet may either be separate or permanently attached to the diaper. When the tape-equipped diaper is made ready for use, the free end of the tape is peeled from the release sheet and secured to the outer thin film side of the diaper after the diaper is positioned on an infant.
The pressure-sensitive adhesive on the tape is formulated to be sufficiently tacky to hold the tape securely in place while in storage, and to insure against disengagement or displacement once the diaper is put on the wearer. Most backing sheets are made of a flexible impermeable film, such as polyethylene, and cost factors dictate that this film be thin and yet provide other needed attributes such as adequate strength, low rattle, high coefficient of friction, etc.
Usually the film is no more than one mil thick. Accordingly, because the film is necessarily thin for economic reasons, if one desires to inspect or adjust the diaper after the tape has been fastened, and in so doing attempts to peel the tape from the film, the usual result is that the thin film will stretch, rupture, and/or tear because of its low tensile load-bearing ability. Further use of the diaper is then impractical, either because the film is torn and weakened, or because adhesive areas of the tape are covered by pieces of film rendering the tape inoperable. When the thin film or tape is damaged by such action, the diaper must be replaced with a new one even though the other components remain substantially undamaged or unsoiled and are otherwise functional.
One suggested solution to the problem is to use tape with a pressure-sensitive adhesive of moderate tack and peel strength which permits peeling the tape off without damage to film or tape and thereby provide refastenability. While the use of a tape with moderate tack permits tape removal with little or no damage to the film or tape components it has been found impractical because the tapes tend to pop off or release spontaneously during use.
Another of the suggested arrangements for solving, or at least ameliorating, this problem is to treat the tape-receiving surface of the film backing sheet itself with a release coating or, alternatively, interposing in the tape-receiving area another tape component having a release-treated surface. Typical patents in this area are U.S. Pat. Nos. 4,020,842 to Richman et al, 4,043,340 to Cepuritis and 4,049,001 to Tritsch. While such release treatment provides easier peel properties, the arrangement also interferes with the initial fastening bond between tape and film, and the security of the bond is compromised. Inadvertent and premature release which might result during use detracts from the desirability of such a system.
Other prior art which has some pertinence to the problem, suggests that the thin film backing be reinforced on its reverse side by laminating to that side a flexible structural material in the area where one end of the tape is secured during manufacture. While emphasis is placed on applying such reinforcement in the area where the tape is first anchored when manufactured, the prior art also broadly suggests reinforcing the area where the free end of the tape is subsequently secured during use.
For example, U.S. Pat. No. 3,867,940 to Mesek et al suggests adhesively laminating to the film a flexible structural material having a higher modulus of elasticity than the film. A high modulus scrim or a polyethylene terephthalate film are specifically suggested as the flexible material for this purpose. However, in such arrangements, it is difficult to obtain a unitary laminate in which the separate layers remain strongly bonded to each other, particularly since a separate bonding agent is used to hold the individual parts together. In addition, complicated process steps are needed to obtain a good laminate. Because of the difficulty in obtaining delamination-resistant bonding, when an attempt is made to peel a strip of high-tack or strongly-adhering tape from the film, the low modulus film tends to stretch and delaminate from the high modulus reinforcement material wherever poor laminar bonding exists causing the film to rupture and/or causing portions of the film to adhere to the pressure-sensitive adhesive. Such situations make refastening impractical. These undesirable results are exacerbated when the reinforcing material has open areas as, for example, when scrim is used.
The present invention overcomes these and other disadvantages by applying to one side of the thin film a layer of compatible hot-melt adhesive, which when applied in hot-melted condition, increases the tear resistance and tensile load bearing ability of the film in the tape securement zones without affecting the ability of the film to securely accept the pressure-sensitive tape.
This above-described uniformly adhered layer of hot-melt adhesive serves to increase the Gurley stiffness of the film-adhesive composite in the selected areas to a stiffness value more closely approaching that of the pressure-sensitive tape itself. The increased stiffness offers a surface which, when the user presses the tape in place, remains firm and keeps the film from deforming in the laminar area. The undeformed surface permits good conformance of tape to the film, and provides a more secure joint between the tape and diaper surface than when a non-stiffened film is the tape-accepting surface, as in the prior art.
In addition, the increased tear resistance of these stiffened areas permits a higher-tack pressure-sensitive tape to be used while assuring that the tape may be peeled from the diaper surface without damaging the film or the pressure-sensitive coating of the tape itself. The improvement thus provides means for reliable refastening, when needed.
In the improved product, the peeling force does not elongate the film beyond its elastic limit during removal and the undeformed surface retains its smooth character which is suitable for reattachment when desired.
Suitable hot-melt adhesives for use in coating the film should have a lower modulus of elasticity than the film, and must have a Ring & Ball softening point lower than the heat distortion temperature of the film so that when applied to the film in hot-melted condition the adhesive will not distort or otherwise thermally degrade the film. The resultant unitary laminate has a structure which is stiffer than uncoated film areas and has a higher elastic limit or yield point then the film alone, and which, as noted above, provides more secure tape attachment. At the same time when the selected areas are subjected to peel stress from a high tack tape the increased tensile load bearing ability in the undercoated areas is such that the film will not delaminate or tear when the tape is pulled away. The film surface is thus left substantially undamaged to permit refastenability when needed.